1. Field of Invention
The invention relates to a touch technique, in particular to a capacitive touch panel detection circuit and a boost circuit thereof.
2. Description of Related Arts
In the 1970s, U.S. Military firstly utilized the technique of touch panel in the military field. Since then, this technique has been gradually turned to the civilian field. With the development of network techniques and the popularization of the internet, people successively witnessed the emerging of the new generation of the touch technique and products which attract the users with their advantages of hardness and durability, rapid response, space saving and easy communication. At present, these relaxing human-computer interaction techniques have be used in many other fields. Except the field of personal portable information products (such as the handwriting input technique of PC, PDA, AV and so on), these techniques have been used in the fields such as information appliance, public information (such as the business inquiries from the departments of e-government affairs, banks, hospitals, electric sectors and so on), computer games, communication apparatuses, office automation equipment, data collection facilities, industrial equipment and so on.
With the rapid development of the consumer electronics such as mobile phones, computers and so on, the touch technique has attracted more and more people and its application fields have been ever-increasing. It has become the most popular input method accepted easily by the users besides the input methods such as keyboard, mouse, handwriting board and voice input. With the technique, the users can operate the host machine only by hands slightly touching the patterns or words displayed on the display screen of the computer; therefore, the human-computer interaction technique becomes belief and to the point. The technique offers great convenience for the users. It is suitable for multi-media inquiries very much. Meanwhile, the human-computer interaction method offers new appearance for the multi-media which are the most attractive and new multi-media interaction equipment. Wherein as multi-touch can be realized by the capacitive touch equipment, functions with more features can be realized. Furthermore, it does not need physical deformation. Therefore, it has the advantages of longer service life and better touch and becomes a rising star in the market of touch equipment with great prospect in the future.
In FIG. 1, the capacitive touch equipment can be divided into self-capacitance detection and mutual capacitance detection. The mutual capacitance detection is such that the changes of coupled capacitance between different rows (columns) are detected when there is touch action. Its principle is as follows: one end of the mutual capacitance is connected with a transmitter and sends out scanning signals, and the other end of the capacitance is a receiver, sends out responses reacted to the scanning signals and calculates the amplitude of the responses. When the changes of the outside world causes the changes of the capacitance values (such as the changes caused by the touch of human body), the amplitude of the responses of the receiver changes, and then the capacitance of the corresponding coordinates can be determined to have been changed. The detection is processed by MCU, and after that, the detection process is completed.
The principle of the self-capacitance detection is such that one end (it is not only a transmitter but also a receiver) of the self-capacitance (the capacitance between wires and the ground) at the same row (column) of a touch apparatus is connected with a detection apparatus, and the other end is connected with the ground. When touch occurs, the capacitance formed by the human body connected with the ground is connected with the self-capacitance on the touch apparatus in parallel, and the equivalent capacitance is increased. At the same time, the response voltage (or response charges and response time) received by the receiver changes, so that touch implemented at the row (column) can be obtained. With the scanning for full screen, the rows and columns positioned by the touch points can be obtained. And then the specific coordinates of the touch points can be obtained.
In order to resist the noise interferences such as RF and increase signal to noise ratio, a relatively sound method is to increase the voltage of the scanning signals. However, many product terminals adopt the power with relatively low voltage to decrease power consumption. Therefore, the boost circuit is needed.
FIG. 2 illustrates a typical boost circuit. It consists of low voltage power supply, an inductor, a Schottky diode, a capacitor, a sequence control circuit and an output-power transistor. A PWM control circuit can send out periodic square waves such as the waveform at A point shown in FIG. 3. When the voltage at A point is in high level, the current of VDD passes through the inductor, a MOS transistor and the ground; furthermore, the current will be increased gradually. At this time, the Schottky diode does not conduct. When the voltage at A point is in failing edge, the MOS transistor is closed. With the inductor, the current is not decreased to 0 immediately, and the voltage at B point is increased rapidly. The Schottky diode does not conduct until the voltage at B point is by 0.2V higher than the voltage at C point. The Schottky diode conducts, and the current is passed to the capacitor. The output voltage at C point is increased, and the voltage at B point is increased correspondingly. Meanwhile, the voltage at C point is detected by the control circuit. If the voltage does not exceed set voltage (VDDH), A point is maintained in the low level, and the capacitor is continuously charged. Otherwise, A point is in high level; the MOS transistor conducts; the voltage at B point is decreased to 0, and the Schottky diode is closed. The voltage at C point is maintained by the capacitor. The loads are charged. The voltage is decreased slowly. The above processes are not repeated until the failing edge of the next half of the cycle is present.
FIG. 3 illustrates a capacitive touch panel detection circuit which is the most popular in the market. This is a mutual capacitance detection circuit consisting of three ICs: a boost circuit, a transmitter circuit and a receiver circuit. With them, the relatively high voltage can respectively be obtained; the scanning signals are sent out to a touch panel, and the scanning signals are responded. Where, an off-chip inductor, a Schottky diode and a capacitor are needed to match with the boost circuit, and the boost circuit is needed to occupy two package pins (that is, boost and sense). The capacitive touch panel detection circuit has the following shortages:
Three ICs are needed to realize the detection functions with the disadvantages of high cost and big occupied area of a circuit board.
The boost circuit needs an inductor, a capacitor, Schottky diode and two package pins. There are many off-chip ancillary components.
An inductive boost circuit has the problem of EMI (electromagnetic interference).